The following discussion is not an admission that anything discussed below is common general knowledge or citable as prior art.
Municipal sewage is often treated in activated sludge wastewater treatment plants. These plants produce treated effluent but also produce waste sludge, which may be primary sludge or waste activated sludge or both. The waste sludge is often treated as a form of solid waste, often referred to as biosolids. To reduce its volume, the sludge may be dewatered, and optionally dried. However, the waste sludge still creates a significant haulage and disposal problem.
As an example, California generated 797,000 dry tons of biosolids from wastewater treatment facilities in 2013. Roughly two thirds of this waste was applied to land and one third was landfilled. California has a goal of diverting 75% of the waste currently being landfilled. This goal applies to biosolids and to roughly 30 million tons of municipal solid waste (MSW) that is landfilled in California annually. For the biosolids, one possible response is to apply more of the biosolids to land. However, while federal regulations control the quality of biosolids that can be land applied to some extent, there is still concern that biosolids can contain Constituents of Emerging Concern (CEC) that are not regulated. CECs include, for example, pharmaceuticals, unregulated industrial and agricultural products, newly registered pesticides, detergents, fragrances, and hormones. There is also concern that biosolids can run off the soil to pollute natural water resources, and many residents simply object to the smell. As a result, several counties have passed bans prohibiting or restricting the application of biosolids to land.
Some wastewater treatment plants have an anaerobic digester that further treats the waste sludge. Anaerobic digestion produces biogas as a result of the biological fermentation of volatile solids (VS) supplied with the feedstock. Typical digesters treating sludge produced in a wastewater treatment plant achieve about 50% VS destruction in 20 day hydraulic retention time (HRT) mesophilic digesters. The digester sludge, or digestate, produced by an anaerobic digester is a combination of inert solids that were fed with the feedstock, recalcitrant volatile solids that could not be degraded biologically, and bacterial biomass that grew as a result of feeding on the degradable portion of the volatile solids in the feedstock. Waste digestate is typically dewatered and optionally dried. This produces another form of biosolids, but the mass of solids that needs to be disposed of is reduced relative to the original waste sludge. Anaerobic digesters are used at only some wastewater treatment plants since, for example, the required land is not always available and the cost of the digester is not always justified, particularly for small wastewater treatment plants.
Organic material can be treated with pyrolysis to generate gas, liquid a solid products. The solid product can be referred to as char, biochar or charcoal. The liquid products include a large number of chemical species including various alcohols, acids, and long chain hydrocarbons. Wood is a preferred feedstock because of its high heat value, low ash content, and availability. Recently developed systems focus on maximizing conversion to a long chain hydrocarbon rich liquid known as “bio-oil” through processes referred to as fast pyrolysis or flash pyrolysis, which occur at moderate temperatures (about 450-550° C.) and short residence times. The short residence times inhibit further decomposition of the bio-oil to other products. However, the bio-oil is diluted with the water from the feedstock and requires treatment to remove impurities in order to recover a liquid fuel. The value of the oil as a fuel does not always justify the cost in treatment and purification.
U.S. Pat. No. 8,877,468 describes a process in which materials containing lignocellulose are treated by pyrolysis under conditions (low temperature and long residence time) that favour the production of a liquid containing organic acids and alcohols. This liquid is suitable for conversion to biogas (primarily methane) in an anaerobic digester. In contrast, bio-oil produced by fast pyrolysis contains significant concentrations of compounds that are toxic to bacteria in an anaerobic digester. U.S. Patent Publication 2012/0322130 describes a system in which liquid pyrolysis products are separated into bio-oil and light oxygenated organic compounds. With the bio-oil removed, the light oxygenated organic compounds can be converted to biogas in an anaerobic digester.
The Applicant is involved in a demonstration project with the Encina Wastewater Authority to treat biosolids by pyrolysis according to a process as described in U.S. Pat. No. 8,877,468. In this demonstration project, dried waste sludge pellets are treated by pyrolysis at about 300-320 degrees C. to produce mostly char and a liquid. The char is applied to land. The liquid is co-digested with liquid waste sludge in an anaerobic digester and thereby converted into biogas. This demonstration project provides two primary benefits. Firstly, char is preferable to biosolids when applied to land. Char is odorless and free of living pathogens. Char sequesters carbon in the soil and improves the physical structure of soil rather than rotting on the land. Secondly, to the extent that some of the biosolids are converted into biogas, a waste product is converted into a source of energy.